An ultrasonic diagnostic apparatus that visualizes body tissues of a patient in a non-invasive manner has a function of measuring the rate of blood flowing through a blood vessel of a patient (flow velocity). Typically, the flow velocity measured based on this function is used for generating a color Doppler image representing the flow velocity distribution in a blood vessel and for generating a Doppler spectrum image representing the time change in flow velocity vary with time. These images are referred to, for example when the flow velocity is checked for a follow-up and when the flow velocity is monitored before and after a surgical operation.
In the measurement of the flow velocity, the flow velocity is calculated based on changes in the frequency of an ultrasonic wave transmitted and received and the angle between the direction in which an ultrasonic wave is transmitted and received and the direction in which the blood flows. The frequency of an ultrasonic wave reflected by the blood flow and then received undergoes a Doppler shift in accordance with the motion of the blood flow. As a result, the frequency is shifted to a frequency higher or lower than the frequency of a transmitted ultrasonic wave. The frequency shift amount is dependent on the motion speed of the blood flow. On the other hand, the angle between the direction in which an ultrasonic wave is transmitted and received and the direction in which the blood flows is dependent on the position and angle of an ultrasonic probe pressed against the patient and on the angle at which an ultrasonic beam is transmitted and received by the ultrasonic probe. Therefore, the calculated flow velocity is greatly dependent upon the angle between the direction in an ultrasonic beam and the direction in the blood flows.